EventBus

A simple pubsub event bus for Ruby applications.

• Gem: https://rubygems.org/gems/event_bus
• API docs: http://rubydoc.info/gems/event_bus/frames
• Source code: https://github.com/kevinrutherford/event_bus

• Simple, global support for the Observer pattern, aka Publisher-Subscriber.
• Publish and subscribe to events throughout your Ruby application.
• Listen for events without coupling to the publishing object or class.
• Subscribe to events using names or regex patterns.
• Works with Rails.
• Works without Rails.
• Completely synchronous (use the event_bg_bus gem for async operation using Sidekiq).

Install the gem

gem install event_bus

Or add it to your Gemfile and run bundle.

gem 'event_bus'

Publish events whenever something significant happens in your application:

class PlaceOrder
  //...
  EventBus.announce(:order_placed, order: current_order, customer: current_user)
end

The event name (first argument) can be a String or a Symbol. The Hash is optional and supplies a payload of information to any subscribers.

(If you don't like the method name announce you can use publish or broadcast instead.)

There are three ways to subscribe to events.

1. Subscribe a listener object:

   EventBus.subscribe(StatsRecorder.new)

   The event will be handled by a method whose name matches the event name:

   class StatsRecorder
     def order_placed(payload)
       order = payload[:order]
       //...
     end
   end

   If the object has no matching method, it doesn't receive the event.

2. Specify the method to be called when the event fires:

   EventBus.subscribe(:order_placed, StatsRecorder.new, :print_order)

   In this case the event will be handled by the print_order method:

   class StatsRecorder
     def print_order(payload)
       order = payload[:order]
       //...
     end
   end

   The first argument to subscribe can be a String, a Symbol or a Regexp:

   EventBus.subscribe(/order/, StatsRecorder.new, :print_order)

3. Subscribe a block:

   EventBus.subscribe(:order_placed) do |payload|
     order = payload[:order]
     //...
   end

   The argument to subscribe can be a String, a Symbol or a Regexp:

   EventBus.subscribe(/order/) do |payload|
     order = payload[:order]
     //...
   end

See the specs for more detailed usage scenarios.

Any exception raised by the event listeners will be swallowed, and will not affect the execution flow. In order to handle errors you can register a global error handler:

EventBus.on_error do |listener, payload|
  #handle the error
end

The supplied block will be called once for each error that is raised by any listener, for any event.

The block will be provided with two parameters, the listener that errored, and the payload of the event. The payload of the event will have an extra error value containing the exception object that was raised:

EventBus.on_error do |listener, payload|
  puts listener.inspect        # => #<Proc:0x007fec9305a500@lib/event_bus_example.rb:10>
  puts payload.inspect         # => {:event_name=>:order_placed, :order_id=>1, :error=>#<StandardError: something went wrong>}
  puts payload[:error].inspect # => #<StandardError: something went wrong>
end

EventBus.subscribe(:order_placed) do |payload|
  raise StandardError, 'something went wrong'
end

EventBus.announce(:order_placed, order_id: 1)

Tested with Ruby 2.1, 2.0, 1.9.x, JRuby. See the build status for details.

(The MIT License)

Copyright (c) 2013 Kevin Rutherford

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.